Ni-Fe-Cr-Mo Alloy

ABSTRACT

The invention relates to an alloy comprising (in mass %) Ni 33-35%, Cr 26-28%, Mo 6-7%, Cu 0.5-1.5%, Mn 1.0-4%, Si max. 0.1%, Al 0.01-0.3%, C max. 0.01%, N 0.1-0.25%, B 0.001-0.004%, SE&gt;0 to 1%, and Fe remainder, including unavoidable impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and Applicant claims priorityunder 35 U.S.C. §120 of U.S. application Ser. No. 13/881,851 filed onMay 29, 2013, which application is a national stage application under 35U.S.C. §371 of PCT Application No. PCT/DE2011/001875 filed on Oct. 20,2011, which claims priority under 35 U.S.C. §119 from German PatentApplication No. 10 2010 049 781.9 filed on Oct. 29, 2010, thedisclosures of which are incorporated by reference. A certified copy ofpriority German Patent Application No. 10 2010 049 781.9 is contained inparent U.S. application Ser. No. 13/881,851. The internationalapplication under PCT article 21(2) was not published in English.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an Ni—Fe—Cr—Mo alloy, especially a modifiedalloy in accordance with EN Material No. 1.4562 as well as its use.

2. Description of the Related Art

The alloy with Material No. 1.4562 has on the average the followingchemical composition (standard values in mass %) Ni 31%, Mn 1.7%, Cr27%, Mo 6.5%, Cu 1.3%, N 0.2%.

DE 32 23 457 A1 relates to an alloy, especially for the manufacture ofhighly loadable pipework of deep boreholes or the like with elevatedresistance to stress corrosion cracking, consisting of C≦0.1%, Mn 3-20%,S≦0.005%, Al≦0.5%, Cr 22.5-35%, W 0-8%, Si≦1%, P≦0.03%, N 0-0.3%, Ni25-60%, Mo 0-4%, Cu 0-2%, RE 0-0.1%, Mg 0-0.1%, Ca 0-0.1%, Co 0-2%, Y0-0.2%, Ti 0-0.5%, wherein Cr (%)+10 Mo (%)+5 W (%) 50% ½ Mn (%)+Ni(%)≧35 (%) 1.5%≦Mo (%)+½ W (%)<4.

From U.S. Pat. No. 5,841,046, a high-strength corrosion-resistantaustenitic non-rusting steel, which contains an effect total (PREN)>55,can be inferred. An alloy of the following composition (in mass %) ispresented: max. 0.08% C, 0.5-12.5% Mn, 20-29% Cr, 17-35% Ni, 3-10%Mo, >0.7% N, up to 1.0% Si, up to 0.02% B, up to 0.02% Mg, up to 0.05%Ce, remainder iron. For chromium contents between 24 and 28%, the nickelcontents are indicated as between 21 and 23%, wherein the effect total(PREN) ranges between 49 and 65. In this state of the art, the extremelyhigh nitrogen content is of significance.

In U.S. Pat. No. 4,824,638, a corrosion-resistant alloy of the followingchemical composition (in mass %) is described: 20.5-32% nickel, 23.5 to27.5% chromium, 4 to 6.7% molybdenum, 0.7 to 3.6% copper, up to 0.09%carbon, up to 1.5% silicon, up to 5% cobalt, up to 0.45% nitrogen, up to1% titanium, up to 0.8% niobium, up to 0.3% rare earths (Ce, La, mixedmetal), up to 2% manganese, up to 1.6% tantalum, remainder iron, whereinthe sum of the nickel and cobalt contents is between 25.5 and 32% andthe chromium content is exceeded by 2 to 6.2%.

From EP 0 292 061 A1, an alloy has become known with (in mass %) 30-32Ni, 26-28 Cr, 0.5-1.5% Cu, max. 2% Mn, max. 1% Si, max. 0.2% Al, max.0.02% C, remainder Fe, including unavoidable admixtures, whichfurthermore also contain 6-7% Mo and 0.1-0.25% N.

The alloy in accordance with EP 0 292 061 A1 was developed in order tobe able to make available a material that is suitable for themanufacture of structural parts that must have a good corrosionresistance, especially to pitting corrosion and/or stress corrosioncracking, in aqueous, neutral or acid media with high chloride ionconcentration. It is also intended to be usable for the manufacture ofstructural parts that must have an erosion rate of less than 0.20mm/year in technical phosphoric acid with a chloride ion concentrationup to 1000 ppm at 100° C. At the same time, it should be suitable forthe manufacture of structural parts that must have a pitting corrosionpotential of at least 1000 mV_(H) at 75° C. and of at least 800 mV_(H)at 90° C. in aqueous neutral media with a chloride ion concentration onthe order of magnitude of 20,000 ppm. It should further be suitable asmaterial for the manufacture of structural parts that must have acritical pitting corrosion temperature of at least 80° C. and a criticalstress corrosion cracking temperature of at least 50° C. in acid mediawith a chloride ion concentration of 50,000 ppm and higher, such as,e.g. in an FeCl₃ solution.

Consequently, this alloy used heretofore has in practice more thansatisfied the expectations placed on it. However, the very high solutionannealing temperature for dissolution of the brittle sigma phase, whichaccording to VdTÜV Material Sheet 509/1, December 2009 version, must lieat 1150 to 1180° C., together with the additional criterion of asubsequent rapid cooling by means of quenching in water or by means ofcompressed air (depending on the wall thickness), in such a way that thetemperature range down to 650° C. is rapidly transited, has been foundto be disadvantageous for this alloy. For the assurance of a flawlessdissolution of the sigma phase even for thicker-walled structural parts,at least the upper temperature of 1180° C. must be used in theindustrial practice. Cases are now known in which such a solutionannealing treatment must be integrated into the manufacturing process,for example in the hot cladding of large sheet sizes in the sandwichpackage or in the hot pressing of thick-walled vessel bottoms. In thisconnection, it has been found that the solution annealing and coolingconditions mentioned in the foregoing then cannot be satisfied to theextent that the high pitting corrosion and stress corrosion crackingtemperatures expected for this alloy are now not attained because of theseparation of sigma phase.

For alloys (UNS 32654/654 SMO) with similar contents of chromium 24-26%and molybdenum 7-8%, the following empirical formula for the dependenceof the sigma solvus temperature on the alloying components is found inthe literature (Rechsteiner ETH Zurich Publ. No.: 10647):

T sigma-solvus=24.6 Cr+6.7 Mn+50.9 Mo+92.2 Si−9.2 Ni−17.9 Cu−230.4C−238.4 N+447 (element units: mass per cent).

Accordingly, the elements chromium, molybdenum, silicon and manganeseraise the sigma solvus temperature; the elements nickel, copper andespecially nitrogen act to lower the sigma solvus temperature.

SUMMARY OF THE INVENTION

The task of the invention is to provide an alloy that satisfies thetechnical requirements described in the foregoing without relinquishingthe advantages of the previous alloy.

This task is accomplished by an alloy with (in mass %)

-   Ni 33-35%-   Cr 26-28%-   Mo 6-7%-   Cu 0.5-1.5%-   Mn 1.0-4%-   Si max. 0.1%-   Al 0.01-0.3%-   C max. 0.01%-   N 0.1-0.25%-   B 0.001-0.004%-   RE >0 to 1%-   Fe remainder, including unavoidable impurities.

Advantageous improvements of the alloy according to the invention can beinferred from the associated dependent claims.

Surprisingly, it has been found that the high solution annealingtemperature range of 1150 to 1180° C. or higher mentioned initially canbe significantly lowered when the nickel content of this alloy isincreased to 33.0 to 35.0 mass %. For an average nickel content of 34mass % in comparison with previously 31 mass %, the solution annealingtemperature range can be lowered by at least 30° C. to at least 1120 to1150° C. Furthermore, it has been found that an increase of themanganese content by increase of the solubility of nitrogen actspositively on the metallurgical stability. Both manganese and alsonitrogen itself act as stabilizers of the austenitic microstructure. Inaddition, manganese binds sulfur which impairs the hot workability ofthe material. Usually, the Material 1.4562 is manufactured with amanganese content of approximately 1.7 mass % on the average. It has nowbeen found that an increase of the manganese content to 1.8 to 2.6 mass% in combination with an alloying of nitrogen facilitates the solutionannealing treatment by the additional austenite stabilization, in thatthe necessary temperature can still be somewhat further lowered and thenecessary time shortened. However, too high manganese contents impairthe corrosion resistance, which is apparent, for example, during themeasurement in the “Green Death” test solution.

It is therefore not obvious to increase the manganese content. Actually,in the laboratory smeltings performed for manganese, it was possible bymetallographic examinations to find the raising effect on the sigmasolvus temperature, but this apparent disadvantage is eliminated byimprovement of the nitrogen solubility in the alloy matrix. According toRechsteiner ETH Zurich, this nitrogen lowers T sigma-solvus and isavailable for the increase of the corrosion resistance to pittingcorrosion in chloride-containing media according to the PREN formula asfollows.

PREN: Cr+3.3 Mo+30 N

The nitrogen solubility increased by the manganese addition definedaccording to the invention leads to a weaker binding of the nitrogen tochromium as a metal nitride. Hereby the effective content of chromiumCr_(eff), which is available for the increase of the corrosionresistance, is increased.

Cr_(eff)=Cr−10 Cr (C+N)

The effect total PREN (=mass % Cr+3.3 mass % Mo+30 mass % N) from theweighted contents of chromium, molybdenum and nitrogen should lie abovea value of 50 on the average for the new alloy just as for the Alloy1.4562. The structural parts manufactured from it should, under theconditions according to ASTM G 28, Practice A, be resistant tointergranular corrosion and in the solution-annealed condition shouldhave an erosion rate of less than 0.5 mm/year. Finally, it should alsobe suitable for the manufacture of structural parts that must be free ofstress corrosion cracking and pitting corrosion under the conditions ofan aggressive proof test with sour gas.

A further feature according to the invention of the modified alloyconsists in the use of rare earths (RE), preferably cerium mixed metal.If these are added in the intended scope, they contribute, by thefurther binding of sulfur in addition to the effectiveness of manganese,to a good processability, especially during hot forming. The contents ofRE, especially cerium mixed metal, lie between 0.001 and 0.1%. Thepreferred range is set at approximately 0.06%.

Cerium mixed metal contains not only cerium but also lanthanum,neodymium, praseodymium, samarium, terbium and yttrium as well as tracesof other rare earth metals.

For the improvement of the processability, especially for the hotforming, it is proposed, starting from a diversely usablenickel-iron-chromium-molybdenum alloy (EN Material No. 1.4562), tooptimize the nickel content and the manganese content. In this way, thesolution annealing temperature of the sigma phase can be significantlylowered, without diminishing the resistance of the alloy to technicalphosphoric acid and other technical acids as well as to pittingcorrosion and stress corrosion cracking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a view of the microstructure of an example of the alloyaccording to the invention; and

FIG. 2 shows Pitting Corrosion Temperatures of various alloy samples asobtained at ASTM G 48-C conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred cases of application of the alloy according to the inventionare cited in the following:

-   -   as material for the manufacture of structural parts that must        have a good corrosion resistance, especially to pitting        corrosion and/or stress corrosion cracking, in aqueous neutral        or acid media with high chloride ion concentration;    -   as material for the manufacture of structural parts that must        have an erosion rate of less than 0.20 mm/year in technical        phosphoric acid with a chloride ion concentration up to 1000 ppm        at 100° C.;    -   as material for the manufacture of structural parts that must        have a pitting corrosion potential of at least 1000 mV_(H) at        75° C. and of at least 800 mV_(H) at 90° C. in aqueous neutral        media with a chloride ion concentration on the order of        magnitude of 45,000 ppm;    -   as material for the manufacture of structural parts that must        have a critical pitting corrosion temperature of at least 80° C.        and a critical stress corrosion cracking temperature of at least        50° C. in acid media with a chloride ion concentration of 50,000        ppm and higher, such as, e.g. in an FeCl₃ solution;    -   as material for the manufacture of structural parts that, under        the conditions according to ASTM G 28, Practice A, are resistant        to intergranular corrosion and in the solution-annealed        condition have an erosion rate of less than 0.5 mm/year;    -   as material for the manufacture of structural parts that are        free of stress corrosion cracking and pitting corrosion under        the conditions of a proof test with sour gas.

The alloy according to the invention can preferably be used for theproduction of strips, sheets, bars and forged parts, pipes and wires,likewise as welding rods.

Roll-clad or explosive-clad structural parts that heretofore were onlydifficult to manufacture with the alloy according to Material No. 1.4562because of the high solution annealing temperature can now bemanufactured more easily due to the lowered solution annealingtemperature.

Table 1 discloses exemplary examples of the alloy according to theinvention, smelted in the laboratory (LB 2151), of a large industrialheat (Nicrofer 3426 hMo) and of an alloy to be attributed to the stateof the art (LB 2149), especially their chemical compositions and testresults.

TABLE 1 LB 2149 LB 2151 Nicrofer 3426 hMo (values in %) (values in %)(135755) Ni 31.78 33.84 33.79 Fe 30.8 (R) 29.52 (R) 29.16 (R) Cr 27.9326.74 26.38 Mo 6.16 6.67 6.88 Cu 1.13 1.27 1.16 Mn 1.54 1.54 1.97 Si0.04 0.04 0.05 Al 0.15 0.04 0.04 N 0.18 0.194 0.21 C 0.022 0.0024 0.007S 0.0046 0.0015 0.002 B 0.003 0.004 0.003 RE 0.07 0.02 0.02 Co 0.14

The laboratory batch LB 2149 has an Ni content outside the claimed Nirange.

Metallography:

The microstructure is free of sigma phase and completely recrystallized.

Mechanical Values of 22 mm Sheet:

Tension Test at Room Temperature: Transverse specimens

22 mm sheet Yield Yield Reduction Nicrofer strength strength Tensile ofarea at 3426hMo Rp_(0.2) Rp_(1.0) strength break #135755 N/mm² N/mm² RmA_(man) % 330 371 708 59

Hardness Measurement HRB 84

Notch Impact: Transverse specimens262 joule (Av)

Corrosion Measurements of Critical Pitting Corrosion Temperature in the“Green Death”:

The investigations of the critical pitting corrosion temperature in theGreen Death test medium showed that the target temperature of 55° C. wasexceeded.

The calculated effective PREN from the chromium, molybdenum and nitrogenlies at PREN=54 for the alloy according to the invention and thereforelies at a numerical value above 50, just as the known Alloy 1.4562.

Critical Pitting Corrosion Temperature According to ASTM G48 C:

In the ASTM G48 C test, samples from the sheet rolled to 22 mm attaineda critical pitting corrosion temperature between 90 and 100° C. Samplesof the Material 1.4562 from a 5 mm sheet as comparison attained amaximum temperature of 95° C. in this test.

Corrosion Test ASTM G 28, Practice A (Intergranular Corrosion):

A value of 0.19 mm/year was obtained as the result, and no intergranularcorrosion was observed in the ground section.

What is claimed is:
 1. A structural part comprising an alloy with (inmass %) Ni 33-35% Cr 26-28% Mo 6-7% Cu 0.5-1.5% Mn 1.0-4% Si max. 0.1%Al 0.01-0.3% C max. 0.01% N 0.1-0.25% B 0.001-0.004% RE >0 to 1% Feremainder, including unavoidable impurities, wherein the structural parthas an erosion rate of less than 0.20 mm/year in technical phosphoricacid with a chloride concentration up to 1000 ppm at 100° C.
 2. Astructural part comprising an alloy with (in mass %) Ni 33-35% Cr 26-28%Mo 6-7% Cu 0.5-1.5% Mn 1.0-4% Si max. 0.1% Al 0.01-0.3% C max. 0.01% N0.1-0.25% B 0.001-0.004% RE >0 to 1% Fe remainder, including unavoidableimpurities, wherein the structural part, under conditions according toASTM G 28, Practice A, is resistant to intergranular corrosion and hasan erosion rate of less than 0.5 mm/year.
 3. The structural partaccording to claim 1, wherein the Ni content (in mass %) is 33.5-34.5%.4. The structural part according to claim 1, wherein the Mn content (inmass %) is 1.5-3.5%.
 5. The structural part according to claim 1,wherein the Mn content (in mass %) is 1.5-3.0%.
 6. The structural partaccording to claim 1, wherein the Mn content (in mass %) is 1.5-2.6%. 7.The structural part according to claim 1, wherein the manganese content(in mass %) is 1.5 to 2.0%.
 8. The structural part according to claim 1,wherein the nitrogen content (in mass %) is 0.14 to 0.22%.
 9. Thestructural part according to claim 1, wherein RE is formed by ceriummixed metal in contents between 0.001 and 0.1%.
 10. The structural partaccording to claim 9, wherein the grand total of RE is max. 0.06%. 11.The structural part according to claim 1, wherein the effect total PREN(=mass % Cr+3.3 mass % Mo+30 mass % N)≧50.
 12. The structural partaccording to claim 1, wherein the effect total PREN (=mass % Cr+3.3 mass% Mo+30 mass % N)≧54.